1. Field of the Invention
The invention relates generally to a passive microwave, short range missile guidance and target seeker system. More specifically, the invention is an improved MICrowave RADiometry (MICRAD) guidance system target seeker that supplies constant azimuth and elevation guidance to a missile seeking a surface target on land or sea by means of an interrelationship of emissions received in a passive multiple beam antenna.
2. Description of the Prior Art
For some time, efforts have been made to develop a off tactical missile (STM) involving a Mach 3 air-to-surface missile utilizing a combination of inertial and microwave radiometry guidance systems. A MICRAD seeker was developed from this effort to provide terminal guidance in azimuth and elevation of an STM type vehicle. In certain situations, early designs were adequate for terminal guidance, which applied to the last few miles of a one hundred mile flight, but in other situations, particularly ones involving sea targets where the exact location of the target was unknown, the seeker detection range was shorter than desired and/or the ultimate search for the target prior to acquisition could not be done as rapidly as desired due to seeker integration time limitations. A need to alleviate these shortcomings led to the Cross-Switched MICRAD Seeker described in this disclosure.
Unlike other types of microwave seekers, a MICRAD seeker, in general, utilizes only thermal black body radiation and/or reflected radiation from the sky as the signals from which it derives guidance information. Targets, which are primarily man-made objects, generally serve as good reflectors of a cold sky and thus contrast rather effectively with a natural (warm) background.
The signals received and utilized by a MICRAD seeker are thermal noise. A MICRAD seeker is able to distinguish noise amplitude from one source to another by differentiating between apparent temperatures, a combination of black body and reflected sky effect, of the various sources viewed. Separating signals from one source from those of another source requires the use of an antenna with a narrow field of view (FOV) obtained by using a large antenna and/or an extra high frequency energy medium. Because missile size constraints allow use of only a small antenna, the narrow FOV, e.g. one or two degrees, is obtained by operating a MICRAD seeker in the millimeter wave region of the electromagnetic spectrum, e.g. 10.sup.2 GHZ.
A seeker is a sensor which has the additional capability of providing directional information for homing guidance. A MICRAD seeker is a seeker that utilizes a radiometer type sensor.
Prior art MICRAD seekers relate primarily to a Dicke type radiometer. A principal feature of the Dicke radiometer is that the input to a receiver is switched back and forth between a sensor antenna and a millimeter wave temperature reference. The reference has a known absolute temperature, or one that can easily be measured, and is shielded from other sources of radiation so that it emits black body signals at a power level that can be calculated by the use of Boltzman's Constant (1.380.times.10.sup.-23 joules/ .degree. K). The load thus becomes a source of reference information to which information from the antenna can be compared as a consequence of the switching action. The aggregate temperature of objects within the antenna FOV can thereby be determined regardless of small changes or drifts in the gain of the receiver.
An essential component of the Dicke radiometer, the millimeter wave Dicke switch, is essentially a single pole, double throw (SPDT) switch that is controlled electronically by a switch driver circuit, which may be regarded as a square wave generator. The switch driver circuit also supplies a switching reference signal to a synchronous detector that follows the receiver. The detector then supplies a bipolar output which typically is positive when the signal from the antenna is higher than the reference load signal, or negative when the antenna signal is lower than the reference load signal. Furthermore, the synchronous detector output becomes more positive or less negative, when an object that has a higher apparent temperature comes within the antenna FOV, and vice versa for a colder object.
Since black body emissions within the millimeter wave portion of the spectrum are extremely weak at normal terrestrial temperatures, the receiver used in a radiometer must have very high gain and must minimize noise signals that arise from sources back of the input. In present day radiometers these requirements are generally met by use of a superheterodyne amplifier involving a low noise local oscillator, a low noise mixer, an IF amplifier, typically centered at several hundred megahertz with a bandwidth, counting both sidebands, that may be three times the bandwidth of the IF amplifier, and a square law detector. The output of the square law detector, usually called video, is either detected noise that is modulated by Dicke switching or is any change in input signal due to scanning action of the antenna, both of which involve frequencies much below those of the IF amplifier. Various output circuits then follow the synchronous detector and control the time period for integration of the signals before a unit of output information is supplied. As will be discussed infra, the integration time .tau. is important because it helps determine how fast the antenna may be scanned when it is necessary to search for a target.
More recent MICRAD seekers improved on the earlier Dicke radiometer in several important areas. In recent MICRAD seekers an antenna with four beams is used; canted slightly up (U), right (R), down (D), and left (L) from the central axis of the antenna. Such an antenna could be implemented by using a simple parabolic reflector with four wave guide feeds in a diamond configuration clustered about the focal point. The feed spacing may be such that the 6 dB boundaries of all beams meet on the antenna axis. This results in a multiple beam pattern projected on the ground or the surface of the sea. Black body and reflected sky radiations from within these beams enter the respective antenna feeds and are processed by the seeker to gain the information needed for weapon guidance; i.e. if a contrasting target is present and if it lies more in one beam than the opposite one, the signals reaching the related feeds will differ and the magnitude and direction of that difference forms the basis for guidance corrections. Another improvement in recently developed seeker systems over prior Dicke radiometers lies in the switching arrangement that follows the antenna and in the use of dual parallel receiving channels, one of which is concerned with the azimuth (R-L) measurements, and the other with elevation (U-D) measurements. The operation of the two channels is identical, so only the azimuth channel will be described herein.
Signals from the R and L antenna feeds go to opposite inputs of an SPDT switch, wherein the center contact of the switch leads to the azimuth receiver. The switch is a ferrite electronic type that is typically operated from one input to the other and back to the first at a rate of several kilohertz. The receiver and synchronous detector which follow and the associated millimeter wave local oscillator and switch driver circuits are similar to their Dicke radiometer counterparts. Switching between multiple antenna feeds rather than from a singular antenna feed to a load as in the Dicke case means that instead of comparing the apparent temperature observed by the antenna to a known absolute temperature, the seeker compares what is seen in the R channel to that in the L channel as a reference and vice versa; i.e. only relative data is developed. The output of the synchronous detector may be positive if the target has a higher apparent temperature than its surroundings and lies predominantly within the FOV of antenna feed R, and will be negative if it lies mainly within the FOV of antenna feed L. However, if the target has a lower temperature, these output polarities will be reversed. The output circuits can properly interpret these results either by being preset for a cold or hot target or by measuring whether a higher or lower output occurs as the antenna is scanned from right to left across the target. This interpretation would be included in the functions of the output circuitry so that target azimuth signals supplied to the missile control system or autopilot have the proper guidance sense.
When used on an STM against land targets, the seeker is mounted on gimbals. The gimbals are controlled by an inertial system which points the seeker antenna at the expected target position when it is time for the seeker to acquire the target in the terminal homing phase. Against sea targets, the lack of precise knowledge of an enemy ship's position, plus the ship's travel between the time of setting the target data into the inertial guidance system and the missile's arrival in the ship's vicinity, will generally require that the seeker search the ocean surface with its antenna beams in order to acquire the target. Such autonomous acquisition involves using the gimbals to scan the surface in a prescribed pattern until a difference in apparent temperature between opposite beams (L-R, U-D) indicates the presence of a target, at which point the searching can be stopped and tracking commenced. If the ship is moving, the apparent radiometric wake may be a dominant factor in the detection and acquisition process.
Although these prior art MICRAD seekers were competent and sufficient for their intended purposes at the time, they possessed inherent shortcomings and limitations. A primary improvement offered by the Cross-Switched MICRAD seeker invention disclosed herein is that it uses the signals available in all four antenna beams essentially 100% of the time versus the 50% time use in previous MICRAD seekers. In effect, either data rate at the seeker output or integration time of the Cross-Switched MICRAD seeker is effectively twice that of earlier seekers. Because it is provided with the increased time utilization, the Cross-Switched MICRAD seeker allows operation against a smaller target, targets with reduced contrast from the background, or targets at increased range in comparison to previous seekers.
In addition there exists a continuing need to provide better counter-countermeasure effective techniques. Since all MICRAD seekers to date look right and left or up and down at different times, such seekers may be thrown into a guidance loop oscillation, or may be caused to break lock by an enemy swept frequency or amplitude modulated jammer. Such jammers cause a very strong signal to appear in the R channel for a short time and then in the L channel, and typically vary their sweep frequency in such manner that it is likely to come into synchronism with a seeker switching rate or an odd subharmonic thereof.
The Cross-Switched MICRAD seeker offers a substantial improvement over prior art MICRAD seekers by using a multiple of radiometric receivers which are cross-switched between opposite channels to permit essentially 100% use of incoming signals in all four antenna beams (R, L, U, D). The Cross-Switched MICRAD seeker retains the stability advantages of a Dicke type radiometer and improves the sensitivity by a factor approaching 2. Another ancillary feature of the Cross-Switched MICRAD seeker is the use of synchronous detector type circuits for gain balancing so as to avoid undesirable output modulations associated with and inherent to the switching.